1. Field of the Invention
This invention relates generally to a current regulating circuit and, more particularly, to a method and apparatus for regulating charging and discharging current between two or more parallel-connected battery packs having varying states of health or states of charge, where the apparatus includes no mechanical switches or relays and uses field effect transistors, a current sensor and a control stage to achieve a desired current through each of the parallel battery packs.
2. Discussion of the Related Art
Electric vehicles and gasoline- or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid vehicles offer several desirable features, such as reduced emissions, reduced usage of petroleum-based fuels, and potentially lower operating costs. A key component of both electric and hybrid vehicles is the battery pack. Battery packs in these vehicles typically consist of numerous interconnected cells, which can deliver significant power on demand for propulsion of the vehicle.
For a variety of reasons, including end of vehicle life, battery packs or sections of battery packs can be removed from in-vehicle service. These battery packs or battery pack sections may have reduced energy capacity (state of health) relative to the capacity when they entered in-vehicle service. Yet, even with a somewhat reduced state of health, an electric vehicle battery pack can still store a considerable amount of energy, and can be used for other applications besides powering a vehicle. A variety of different post-vehicle-life uses for such battery packs have been proposed, including using the battery packs in Community Energy Storage (CES) systems.
CES systems store energy for a small community, such as a residential subdivision or a commercial or industrial complex. CES systems typically serve to augment power available from the utility grid, store locally-generated energy from sources such as solar and wind, and provide a backup energy source in the event of a failure of power from the utility grid. Post-vehicle-life battery packs from electric vehicles can be used in CES systems, but the connection of multiple battery packs in parallel is complicated by variations in the state of health of sections of the battery packs. This variation is problematic because multiple battery packs in parallel will charge and discharge at different rates if the battery packs have different capacities, which they are likely to have in a post-vehicle use scenario. If current is not regulated, the differences in charging and discharging rates between the battery packs may cause excessive charging or discharging of some battery packs or cells, resulting in damage.
Various current regulating devices are known in the art, ranging from simple resistive balancers to the transistorized circuit disclosed in U.S. Pat. No. 4,290,007 issued to Fisher et al. While the Fisher et al. device overcomes some disadvantages of resistive balancers and earlier transistor circuits, current flow through the transistor is intended to be uni-directional. Use of a DPDT switch is required to appropriately connect the transistor between the battery and the supply/load to keep the transistor forward biased during use, to maximize the amount of current the transistor can control. If the transistor is reverse biased, the amount of current it can control is severely limited. A method and device are needed for seamlessly regulating both charging and discharging current through each battery pack in a multiple battery pack parallel-connected system.